This invention relates to plastic sheathed cables, particularly cables having an outer sheath which is designed to have flame retardant properties.
Electric cables such as telephone cables are required in some circumstances to have flame retardant sheaths so that in the event of fire they do not readily burn. The cable is subject to a flammability test as laid down by the cable industry in Great Britain, and this consists of holding the cable in a flame for 15 seconds to see if it burns or not. Flame retardancy can be achieved by flame retardant additives. A typical example of a flame retardant additive is sold under the trade name Hytrel HTR 4450.
However, when the flame retardant cable sheath is subject to the flammability test, although the cable sheath does not burn, it will nevertheless melt and may expose the insulation of inner conductors which may not be flame retardant and which will thus burn.